Add sparkline plots prototype

[mbelak-dtml]

Add a prototype displaying a multiple time series dataset in sparklines: Each entity in a separate small plot.
This visualization only works for a small number of lines.

Support both column-wise and entity-wise orientation:

column-wise orientation displays separate plots for each column, with each plot containing all the entities,
entity-wise orientation displays separate plots for each entity, with each plot containing all the columns.

[mbelak-dtml]

Depends on #29 for data to use during development.